WHY DID the Soviet Union, in its first real encounter with the Carter administration, reject two U.S. proposals for a new strategic arms agreement in such definitive fashion?

There are plenty of contending theories. Explanations for the hard-nosed decision range from Soviet displeasure with President Carter's public style of diplomacy to the claim that the administration's proposals were so innovative that the Soviet leadership was unprepared to deal with them seriously during Secretary of State Cyrus Vance's short visit to Moscow.

As plausible as these various explanations are, none really address Soviet Foreign Minister Andrei Gromyko's charge that the proposals tabled by the administration "pursued the aim of getting unilateral advantages for the U.S.A. at the detriment of the Soviet Union." In fact, as the details of the two proposals emerge, it becomes difficult to resist the conclusion that, from the Soviet perspective, the U.S. offers were unbalanced and probably non-negotiable.

For a start, one of the proposals had already been rejected by the Soviet Union in discussion with the Ford administration last year and there was little reason to expect a more favorable response the second time around.

This was to accept the mutual ceiling of 2,400 on balistic missiles and long-range bombers agreed to by President Ford and Soviet Communist Party chief Leonid Brezhnev at their 1974 summit at Vladivostok and to defer to a later round of talks the troublesome questions of whether and how limits would be placed on the Soviet Backfire bomber and U.S. cruise missiles.

The Soviet Union is deeply concerned about the U.S. cruise missile program and has repeatedly warned that, without constrains placed on its deploment, a new agreement cannot be signed. Because these low-flying, highly accurate drone aircraft are still under development, it is not surprising that Brezhnev was unwilling to postpone their inclusion in a new accord.

However, Soviet opposition to the U.S. proposal for reducing ballistic missile and bomber forces is more difficult to understand. This plan - the administration's "preferred option" - would have reduced the Vladivostok ceilings for strategic forces to between 1,800 and 2,000 and would have also cut the number of launchers equipped with multiple, independently targeted reentry vehicles (MIRVs) from 1,320 to between 1,100 and 1,200.

Significantly strict range limits would have also been placed on cruise missiles. The MIRV Problem

THEN why did the Soviet Union turn it down? Despite the argument of some, the answer does not seem entirely due to Soviet unwillingness to countenance reductions. For one thing in negotiations with then Secretary of State Henry Kissinger in January 1976, the Soviet government itself suggested that the forces of the two sides be reduced by a few hundred from the levels outlined at Vladivostok.

The answer, instead, appears to stem from the fact that the "deep cuts" proposal was not just designed to reduce strategic forces generally, but also sought to single out a special category of weapons for reduction - land-based missiles with MIRVs.

For over a decade, international ballistic missiles (ICBMs) housed in concrete-hardened underground silos have made up the largest single component of both U.S. and Soviet strategic forces. During the 1960s, these systems were considered, for the most part, to be invulnerable to attack.

But with the widespread deployment of multiple warheads and the improvement of missile accuracies in the 1970s, many analysts in the West have grown increasingly concerned over the ability of land-based missile to survive attack. Some are particulary worried over the possibility that the new generation of MIRVed Soviet ICBMs now undergoing deployment - the SS-17, 18 and 19 missiles - might be capable of destroying most, if not all, of the U.S. land-based Minuteman force in a first-strike.

As a result, many strategists have called for the two superpowers to move their nuclear forces to sea in the form of ballistic missile submarines.

It appears that this was what the reductions proposal was, in part, designed to do. In addition to lowering the overall ceilings for strategic forces and launchers equipped with MIRVs, the proposal reportedly also called for two additional ceilings on land-based missiles: a limit of 550 for MIRVed ICBMs and a 50 per cent reduction in numbers of large-payload, "heavy" ICBMs (which only the Soviet Union deploys).

These measures would have had a significant impact on existing and future Soviet ICBM capabilities. To began with, the Soviet Union places a far greater reliance on the ICBM than does the United States. It would not be an exaggeration to claim that land-based missiles make up the backbone of Soviet strategic power: Soviet ICBMs now number over 1,500 (almost 500 more than the United States has) and over three-quarters of the payload that the Soviet Union could use in a nuclear attack is tied up in these systems.

At the time, Soviet bomber and submarine-launched missile capabilities are correspondingly weak. Soviet intercontinental-range bombers number less than 150 and most are 20 years old.

And while the Soviet Union is rapidly building up its sea-based missile forces, for technical and geographical reasons it is still sensitive about becoming too dependent on a sea-based deterrent. For example, while the United States keeps roughly half of its missile submarines at sea and in range of targets in the Soviet Union, only 15 per cent of the Soviet submarine missile force is estimated to be at sea at any one time.

The U.S. proposal for reductions, then, undoubtably raised troubling questions for Moscow. Under the Vladvostok guidelines, the Soviet would have been permitted missiles with its new generation of MIRVed missiles. Under the reductions proposal, however, the Soviet Union would have been force to cut back on its plan for ICBM replacement and modernize many of its sea-based missiles instead.

From the U.S. peroective, this would have been an important step towards greater strategic stability because it would have reduced the ability of either side to carry out a disarming strike against the other's ICMBs. But seen from Moscow, the U.S. proposal was an attempt to channel superpower strategic competition into an area geographically and technologically dominated by the United States. How To Negotiate

THUS, the administration's two proposals were almost bound to run into trouble. But this does not meant that they should not have been tabled. Particularly in the case of Carter's proposal for mutual reductions, the espisode in Moscow tells us something about Soviet attitudes towards arms control and the future of the negotiations generally.

First, the Soviet Union does not, as some have argued, seem opposed to the general idea of reductions. It was the conditions under which they were to be imposed that bothered the leadership.

In wider terms, while both sides remain committed to reaching a new agreement before October, neither side can be expected to enter into an accord that is seen to run counter to its own definitions of what constitudes deterrence and nuclear "sufficiency." Thus, a reductions proposal that allowed each side greater flexibility in structuring its strategic posture might enjoy a better chance of success.

Second, wile U.S. negotiators should not be timid in trying out new ideas, the "shock effect" of new proposals should be minimized. The Soviet leadership appeared honesty surprised that the administration's "preferred option" strayed so far from the understandings worked out at Viadivostok. However, Gromyko's threat to re-introduce the sentitive issue of U.S. aircraft based in Europe into the negotiations - after they had been left out of the Vladivostok ceilings - shows that this is a game that both sides can play.

If the Soviet Union does raise the issue of U.S. "forward-based systems" (U.S. nuclear-capable aircraft in Europe and aboard carriers) at the talks, the result would be serious. It would not only complicate the NATO-Warsaw Pact discussions in Vienna over force reductions in Central Europe, but it would also create new strains in relations between the United States and its allies in Europe.

Third, because the two sides view their nuclear relationship from different perspectives, proposals that require a basic agreement on then*ature of strategic problems are unlikely to get very far.

The U.S. reductions proposal was clearly designed to cope with the growing problem of land-based missile vulnerability. But this is a problem that the Soviet Union doesn't even appear to worry about. The strength of the Vladivostok approach, on the other hand, is that it provides the "elbow room" for each side to react to its own set of strategic concerns. Seeking Too Much

THE SOLUTION to some strategic problems, moreover, may lie outside the realm of arms control negotiations. The United States, for instance, could probably solve the missile vulnerability problem by taking unilateral steps - phasing out the Minuteman ICBM force and replacing it with additional sea-launched missiles or by deploying a mobile ICBM force.

In addition to being expensive, however, either of these two steps would introduce new complexities into the U.S.-Soviet arms control dialogue. But there is no simple way to resolve this dilemma. In some cases, it will be preferable to forego unilateral intiatives that are likely to make negotiations more complex. It is important to recognize, however, that arms control is not just concerned with obtaining agreements and that the objectives sought for arms control can sometimes be more easily achieved by other means.

The administration's encounter with Brezhnev indicates, then, that there can be a danger in asking more from arms control than it can deliver. There are some problems that formal negotiations, at least at the present, are unsuited to resolve.Carter obviously wants to build up a strong domestic constituency for disarmament. But exaggerating what the talks can accomplish is likely to make negotiations more difficult and could also generate expectations that a new agreement - if it comes - will inevitably disappoint.